Generally, it is efficient that channel information is fed back for efficient communication. For this, downlink channel information is sent in uplink and uplink channel information is sent in downlink. This channel information is called a channel quality indicator (hereinafter abbreviated CQI). The CQI can be generated in various ways. For instance, the CQI can be generated by one of a method of quantizing and uploading a channel status, a method of calculating and uploading SINR, a method of indicating a state for applying a channel actually like MCS (Modulation and Coding Set) scheme and the like.
In the various CQI generating methods, it is in general that the CQI is actually generated based on MCS. This is explained in detail as follows. First of all, there is CQI generation for transmission scheme such as HSDPA in 3GPP system and the like. Thus, in case that CQI is generated based on MCS, the MCS particularly include a modulation scheme, a coding scheme and a corresponding coding rate. As the modulation and coding schemes are changed, the CQI has to be correspondingly changed. Hence, at least one CQI is necessary per codeword.
If MIMO is applied to a system, the number of necessary CQIs may be increased. In particular, since MIMO system generates multi-channels using a multi-antenna, several codewords are usable at the same time. Hence, several CQIs should be used correspondingly. Thus, in case that a plurality of CQIs are used, a size of corresponding control information increases in proportion.
FIG. 1 is a conceptional diagram for generation and transmission of CQI.
Referring to FIG. 1, a terminal or UE (User Equipment) 100 measures a downlink quality and then reports a CQI value selected on the basis of the measured downlink quality to a base station (BS) via uplink control channel. The base station 200 performs downlink scheduling (e.g., terminal selection, resource allocation, etc.) according to the reported CQI. In this case, the CQI value may include one of SINR (signal to interference and noise ratio) of channel, CINR (carrier to interference and noise ratio), BER (bit error rate), FER (frame error rate) and the like. And, the CQI value may include a value resulting from converting one of them to transmittable data. In case of MIMO system, information reflecting a channel status such as RI (rank information), PMI (precoding matrix information) and the like can be added to the CQI value.
In a mobile communication system, link adaptation is adopted to maximumly use channel capacity given to a channel. The link adaptation provides a method of controlling MCS (modulation and coding set) and transmission power according to a given channel. In order for a base station to perform the link adaptation, a user has to feed back channel quality information to the base station.
If a frequency band used by a system exceeds a coherence bandwidth, a channel is abruptly changed within a system bandwidth. Specifically, in a multi-carrier system such as orthogonal frequency division multiplexing (hereinafter abbreviated OFDM) system, a plurality of subcarriers exist within a given bandwidth. As modulated symbols are carried by each of the subcarriers, optimal channel transmission means that channel information of each subcarrier is transmitted. Hence, a feedback size of channel information in a multi-carrier system having a plurality of subcarriers is abruptly increased.
Moreover, in case that channel information on a specific frequency band is selectively transmitted to reduce the above-mentioned increase of the feedback size of the channel information, information on a position of a selected frequency band should be transmitted in addition.